My wife, a kindergarten teacher surrounded by 5 year olds on a daily basis, thinks 15, for a healthy young man to take on. She gives herself 8-10 before she goes down.
Her argument is that when they surround her to hug her, she's already knocked off balance. Change these kids into little fighters out for blood, and she thinks one would go down pretty quickly. Then if they start kicking your head / suffocating you, you'll be done pretty quick.
I'm thinking slightly higher: 20-25 - mainly because I can keep my distance and take out several before they close and take me down.